There has been known, as a filter used in a communication device such as a mobile phone, a ladder-type filter in which piezoelectric thin film resonators are connected in a ladder form. The piezoelectric thin film resonator has a structure designed to have a lower electrode and an upper electrode sandwiching a piezoelectric film. A duplexer and a module including a plurality of filters may be installed in a communication device.
In the ladder-type filter, a series resonator and a parallel resonator are configured to have different resonant frequencies. As a technique for forming the frequency difference, it has been known to pattern a frequency control film formed on the upper electrode to make the masses of the frequency control films of the series resonator and the parallel resonator different from each other as disclosed in, for example, International Publication No. 2007/000929. To form the frequency difference and reduce spurious, it has been known to configure the resonators to have an identical pitch interval of and different areas of protrusion patterns or hole patterns formed on the frequency control film as disclosed in, for example, Japanese Patent Application Publication No. 2011-71913.
The conventional technique, however, has room for improvement in reducing spurious.